I had a SD-B for 25 years and reloaded well over a 130,000 + rounds on it. After competing in 3 different style of shooting competitions all at the same time, the SD-B was just not doing it for me. It was hard to load the massive quantities I required on and was also limited to certain pistol calibers. The Dies are also proprietary and no other brand of Die can be used on he SD-B. I also found that the ergonomics on the SD-B are just not meant for large quantities and long hours of operation. BTW, Dillon sent me a bunch of free replacement parts so I could rebuild a lot of the worn ones and the second rebuild was a major one done by them - for free. Can't beat their customer service!!! The 650 and 750 will go much farther before requiring any parts replacements & overhauls!!
About 15 years ago I sold my SD-B and upgraded to an XL650 which can use any standard Dies. It is way way faster, much easier to use, has much better ergonomics, can crank out ammo much faster and can also reload many rifle calibers. There are many more attachments like a case feeder, bullet feeder, loading trays and a bunch of add on after market accessories as well. With the roller handle accessory my hands do not get torn up, the action is so much more fluid and easier to operate and automatic case feeding is a pleasure!
After owning a SD-B for 25 years and an XL650 for 15+ years I would advise anyone who can to get the XL750 which has taken the place of the XL650. The 750 has a few upgrades over the 650 but is a similar type machine super heavy duty, accurate, precise and a pleasure to use. While the SD-B is an OK machine, it is just too limited, as far as I am concerned and meant basically for small quantity reloading use. Yes, I learned a lot about reloading on the SD-B but regretted not getting the 650 sooner. IMHO, the SD-B is very quickly outgrown. Again, the SD-B will get most of the job done for handguns but at a heavy price on comfort, speed, effort, proprietary only dies and limited accessories availability.
BTW, I have never owned a Dillon 550 but have used a few at friend's houses. IMHO, the 650 and 750 blow the 550 away! Just my opinion of course.